华北克拉通北缘晚中生代火山岩Sr-Nd-Pb同位素填图及其构造意义  

Sr-Nd-Pb isotope mapping of Late Mesozoic volcanic rocks across northern margin of North China Craton and implications to geodynamic processes

作　　者：周新华[1] 张国辉[1] 杨进辉[1] 陈文寄[2] 孙敏[3] 

机构地区：[1]中国科学院地质与地球物理研究所,北京100029 [2]中国地震局地质研究所,北京100029 [3]香港大学地球科学系

出　　处：《地球化学》2001年第1期10-23,共14页Geochimica

基　　金：国家自然科学基金资助项目!( 49733110);香港 RGC资助项目! (HKU7110/97P); 科学技术部资助项目! (G1999057704)

摘　　要：辽宁西部广泛分布着晚中生代火山岩，横跨华北克拉通和兴-蒙造山带两大构造单元，一般认为它们是以在这一地区发育的两条主要断裂——西拉木伦河断裂和赤峰-开源断裂为界。这些火山作用曾被认为是中生代伊泽奈崎板块西向或西北向消减作用的结果。在岩性上它们主要以中酸性岩石为主，玄武岩等中基性岩石较少。为查明下伏岩石圈对这些岩浆作用成因的影响，对其中的 SiO2含量 < 60％的中基性火山岩进行了详细的 Sr- Nd- Pb同位素研究。结果显示，以西拉木伦河断裂和赤峰-开源断裂两侧为界，两侧火山岩的同位素特征存在显著区别。南区有明显的 EMI特征并具 EMI- PM混合趋势，而北区则显示了原始或略亏损的特征。南北两区之间的过渡带（介于西拉木伦河断裂和赤峰-开源断裂之间）则表现了同位素组成上相应的过渡特征。这一地球化学观测与已有的地质和地球物理资料颇为吻合。基于新生代幔源岩石化学反演的中国东部陆下地幔化学区划研究揭示了克拉通下 (subcratonic)岩石圈地幔与 EMI存在着密切关系。本研究所提供的地球化学证据说明了采用中生代中基性岩类作为类似研究途径的可行性，并进一步提出对华北克拉通边缘中生代火山作用成因另一种可能的解释。Late Mesozoic volcanic rocks are extensively spread on both sides of northern margin of the North China Craton (NCC), where is the boundary between an Archean craton to the south and a Paleozoic orogenic belt, Xing an- Mongolia Orogenic Belt (XMOB), to the north. Two major east- west trending faults, Kaiyuan- Chifeng Fault (KCF) and Xar Moron He Fault (XHF) develop in this margin zone as a boundary between two tectonic domains. The volcanism has been thought to be the magmatic response to the west- , or northwest- dipping subduction of Izanagi Plate and covers a wide range in lithologies, from rhyolites, dacites, trachyte, andesites to basalts. To clarify the role of underlain lithosphere in magma genesis, a systematic Sr- Nd- Pb isotope survey has been taken on these selected samples that contain SiO2< 60％ , i.e. basalt, trachyandesite and basaltic andesite. The results show a distinctive and contrasting pattern of isotope signatures on two sides of the boundary faults, KCF and XHF. A remarked EMI signature and EMI- PM mixing trend feature the isotope system of basaltic volcanism on the southern side, the North China Craton, whereas the primitive to slightly depleted signature dominates that of basaltic volcanism on the northern side, the Paleozoic Orogenic Belt (XMOB). A clearly transitional and variable signature has been shown in the area between two parallel EW trending faults, i.e. KCF and XHF. This picture is quite consistent with the available geological and geophysical observations. A chemical zoning of subcontinental mantle beneath eastern China (Zhou and Zhu, 1992) has shown the EMI affinity of sub- cratonic mantle based on a chemical inversion study of the Cenozoic mantle- derived rocks. However, a positive argument is presented to show that the magma sources of Mesozoic volcanic rocks in the region have been highly influenced by the underlain lithosphere although the magma has been evolved to different degrees. Furthermore, the results would support an alternative interpretation, reactivation of

关 键 词：晚中生代 火山岩 华北克拉通 古消减带 地质填图 岩浆作用 地质构造 锶-钕-铅同位素 

分 类 号：P588.14[天文地球—岩石学] P285.1[天文地球—地质学]